This invention relates to rear projection systems, and more particularly to compact rear projection systems using polarization sensitive designs.
Rear projection systems typically comprise some type of image generation source, optics to enlarge and direct the image and a transmission screen for displaying the enlarged image. The image source can be of many different types, including cathode-ray tubes and LCD projectors. In simple systems, the optics generally includes a lens, such as a combined convex glass lens element and a methacrylic resin lens element, and a turning mirror for directing the image toward the screen. The transmission screens generally include diffusing material, lenticular lens sheets and Fresnel lens sheets, and are intended to project a wide image with uniform brightness.
In one rear projection system configuration, the image source is positioned behind the transmission screen and directed at an angle away from the screen and toward the turning mirror. The image source provides a small, bright image to the projecting lens, which enlarges the image and directs it to the reflective surface of the turning mirror. The turning mirror reflects the image to the transmission screen for transmission to the viewing audience.
The depth dimension of rear projection systems like the one described above is constrained by the angle of incidence on the screen""s Fresnel lens. To make a compact rear projection package, a short focal length lens is required. A decreasing focal length resulting from decreasing cabinet depth increases the field of view as measured at the screen. As the field of view increases, the angles of incidence in air and within the Fresnel lens eventually approach the critical angle, causing transmission to drop to zero. Even before the angle of incidence approaches the critical angle, the perpendicular and parallel polarization transmission coefficients diverge. A divergence in polarization transmission coefficients results in image distortions, such as non-uniformity in brightness across the screen.
Recently there has been research into the use of birefringent optics in optical systems, including their use to produce polarization-dependent elements and multilayer polymer mirrors. These elements reportedly exhibit extinction ratios as high as 300:1, wide-angle acceptance, and bandwidth selectability in the visible range. Nevertheless, currently available materials are polarization orientation sensitive, and have not experienced widespread use in rear projection systems.
A rear projection system according to the principles of the invention provides for reduced cabinet depth by folding the optical path with polarization-dependent reflectors and retardation material. In one aspect, the projector outputs linearly polarized light to a quarter-wave polarizing sheet. The quarter-wave sheet circularly polarizes the projector output. A turning mirror then directs the projected light to a screen having a layer of quarter-wave material, a polarization-dependent reflecting film and an off-axis Fresnel lens. The circularly polarized light, upon passing through the screen""s layer of quarter-wave material, becomes linearly polarized and is reflected by the polarization-dependent reflecting film. After reflection, the screen""s quarter-wave material again circularly polarizes the light. The circularly polarized light is directed back to the screen by a second mirror, where the screen""s quarter-wave material causes the light to again become linearly polarized. The polarization-dependent film passes the linearly polarized light to the off-axis Fresnel lens. In this manner, the optical path is folded such that the incident angle on the Fresnel lens is a projection quality angle (for example, an angle less than the Brewster angle).
In another system according to the invention, the optical path includes a turning mirror that directs a scanning point source projection to a quarter-wave retardation film bonded to a birefringent mirror. The quarter-wave material manipulates the polarization state and the mirror reflects the light to a polarization-dependent reflector. The reflector initially reflects the light. Once the light traverses the quarter-wave material and mirror a second time, the polarization state is transmissive. A collimator and light control film compensate for the effects of any light leaking through the polarization operative elements.